This invention relates to a device for following the centre line of a railway track.
In application Ser. No. 899,969 of Raymond Ralph Lund filed on Apr. 25, 1978 there is described a machine for removing existing rails and laying new rails on the old ties. Various operations have to be carried out after the lifting of the old rails before the new rails are laid down. For example, it is necessary to pick up the old tie-plates, sweep or crib the ballast clear of the old ties at the tie plate locations, adze the ties at these locations and drop at these locations tie-plates, either recycled or different ones, to receive the new rails. As described in the above patent application these operations are carried out automatically by tie-plate pick up means, crib sweeping means, adzer and tie-plate dropper means, respectively, mounted on the machine. It should be appreciated that all of the above operating means are provided for each rail and that e.g. the two tie-plate pick up means are mounted the rail gauge distance apart. The same is true for the other pairs of operating means.
As indicated in the prior application, the various pairs of operating means may be mounted for transverse movement on their frames under the control of a centre line follower such as a photocell which detects or follows a centre line previously painted on the track. This permits the pairs of operating means to be correctly positioned laterally at the rail locations even when the machine is negotiating a curve on the track.
While the photocell centre line follower described would work satisfactorily, it is difficult to ensure a "true painted line of predetermined width particularly where the paint is being applied to the ballast between the ties and, if the line varies in width the centre line follower would tend to oscillate even on straight track."